Producing Acoustic ‘Frozen Waves’: Preliminary Results

In this paper we show how appropriate superpositions of Bessel beams can be successfully used to obtain arbitrary longitudinal intensity patterns of nondiffracting ultrasonic wavefields with very high transverse localization. More precisely, the method here described allows generating longitudinal acoustic pressure fields, whose longitudinal intensity patterns can assume, in principle, any desired shape within a freely chosen interval 0 z L of the propagation axis, and that can be endowed in particular with a static envelope (within which only the carrier wave propagates). Indeed, it is here demonstrated by computer evaluations that these very special beams of non-attenuated ultrasonic fields can be generated in water by means of annular transducers. Such fields “at rest” have been called by us Acoustic Frozen Waves (FW). The paper presents various cases of FWs in water, and investigates their aperture characteristics, such as minimum required size and ring dimensioning, as well as the influence they have on the proper generation of the desired FW patterns. The FWs are solutions to the wave equation that can be used in many applications, like new kinds of imaging devices, acoustic tweezers or bistouries, and especially ultrasound medical apparatus for the destruction of tumor cells without affecting the preceding and subsequent (and surrounding) tissues.